Introduction to Elevation of Boiling Point
The elevation of the boiling point refers to the rise of a solvent's boiling point upon the addition of a solute. The resulting solution has a higher boiling point when a non-volatile solute is applied to a solvent than that of the pure solvent. The boiling point of sodium chloride (salt) solution and water, for example, is higher than that of pure water.
The elevation of boiling points is a colligative property of matter, i.e., it depends on the solvent-to-solvent ratio but not on the identity of the solute. This means that the elevation of a solution's boiling point depends on the amount of solution applied to it. The higher the solute concentration in the solution, the greater the elevation of the boiling point.
Boiling Point Elevation
The vapor pressure of a solvent can decrease when a solution is applied. This occurs because of the solute displacement of solvent molecules. This means that some of the solvent molecules on the liquid's surface are replaced by the solvent; both electrolytic and non-electrolytic solutions will occur. The lower number of solvent molecules on the surface means that less can evaporate, thus reducing the vapor pressure. A higher temperature is needed for the vapor pressure to equal the ambient pressure, and a higher boiling point is observed.
A graph describing the elevation at the boiling point of water when sucrose is added is given above. At a pressure of 1atm, pure water boils at 100\[^{\circ}C\]. However, in water, a 10-molal solution of sucrose boils at around 105\[^{\circ}C\].
Why Does Boiling Point Elevation Occur?
The temperature at which its vapor pressure is equal to the pressure of its surrounding atmosphere is the boiling point of a liquid. Non-volatile liquids do not evaporate quickly and have very low vapor pressures (assumed to be zero). The vapor pressure of the resulting solution is lower than that of the pure solvent when a non-volatile solute is applied to the solvent.
Therefore the solution must be supplied with a larger amount of heat for it to boil. The boiling point elevation is this rise in the solution's boiling point. A rise in the concentration of the added solution is followed by a further decrease in the solution's vapor pressure and a further increase in the solution's boiling point.
A temperature graph of pressure v/s detailing the boiling point elevation of a solution is given below.
Here, ΔTb represents the elevation of the solution's boiling point. It can be observed from the graph that:
The solution's freezing point is lower than that of the pure solvent (freezing point depression).
The solution's boiling point is higher than that of the pure solvent.
Note: The liquid's boiling point often depends on the pressure of its surroundings (which is why water boils at temperatures lower than 100\[^{\circ}C\] at high altitudes, where the surrounding pressure is low).
Boiling Point Elevation Formula
The boiling point of a non-volatile solute containing solution can be expressed as follows:
Boiling point of solution = pure solvent boiling point + elevation of the boiling point.
The boiling point elevation (ΔTb) is proportional to the solute concentration in the solution. The following equation allows it to be measured.
ΔTb = i*Kb*m
Where,
It is the Van’t Hoff factor.
Kb is the ebullioscopic constant.
m is the molality of the solute.
It is important to remember that when the solute concentration is very high, this formula becomes less accurate. Also, this formula for volatile solvents does not hold true.
In terms of\[^{\circ}C\] /molal, or\[^{\circ}C\].kg.mol-1, the ebullioscopic constant (Kb) is also expressed. Below the Kb values for some common solvents are tabulated.
Kb Values for Some Common Solvents
With the support of the boiling point elevation formula, the degree of dissociation of the solute and the molar mass of the solute can be measured.
The Relationship Between Boiling Point Elevation and Vapor Pressure
In terms of vapor pressure, boiling point elevation can be clarified. Vapor pressure is defined as the pressure exerted at a given temperature by a vapor in thermodynamic equilibrium with its condensed phases. It is simply a measure of the ability of the solvent molecules, in layman's words, to escape by entering the gas phase. When the vapor pressure is equal to the air pressure, a liquid boils.
Boiling Point - The boiling point of a liquid in its purest form. The liquid can boil when the vapor pressure of the liquid equals the ambient pressure.
What do we Mean by Boiling Point Elevation?
By the term boiling point elevation we mean the increase in the boiling point of a solvent after a solute is added. So when any non volatile solute is added to the solvent then the result solution has a higher boiling point that the pure solvent. Like for instance the boiling point of the solution chloride that is salt coupled with water is prominent than the pure water. However the boiling point elevation is a colligative property of matter as it depends on the solute to solvent ratio and not on the identity of the solute. This means that the elevation in the boiling point of solution is dependent on the amount of solute that gets added. Hence the greater the concentration of solute is there in the solution then greater will be the boiling point elevation.
Why does Boiling Point Elevation happen?
However the boiling point of a liquid is perhaps the temperature at which the vapour pressure is equal to the pressure of its nearby environment. Moreover non-volatile substances do not actually undergo evaporation and have low vapour pressures which we can assume to be at 0. However when the non-volatile solute is given to the solvent then the vapor pressure of the resulting solution is lower than the pure solvent. Henc the increased amount of heat should be supplied to the solution so that it can boil. This increase in the boiling point of the solution is known as boiling point elevation.
However if this is put in vapour pressure terms the liquid boils at the temperature when the vapor pressure becomes equal to the surrounding pressure. However for the solvent if the solute is present it will decrease its vapor pressure by dilution.
What is the Use of Boiling Point Elevation?
The boiling point election and formulas used for boiling point elevation can be used to weigh the degree of dissociation or the molar mass of the solute. However this type of measurement is known as ebullioscopy which is a Greek term for boiling viewing. However the cryoscopic constant which says that the freezing point depression is larger than the ebullioscopic constant. But the freezing point is easy to measure with accuracy and is commonly used in cryoscopy.
FAQs on Elevation Boiling Point
1. What is elevation in boiling point? Explain with an example.
Elevation in boiling point is the phenomenon where the boiling point of a pure solvent increases upon the addition of a non-volatile solute. This occurs because the solute particles reduce the solvent's vapour pressure, meaning a higher temperature is needed for the vapour pressure to equal the atmospheric pressure. A common example is adding salt to water; while pure water boils at 100°C at standard pressure, salted water boils at a slightly higher temperature.
2. What is the formula to calculate the elevation in boiling point?
The formula for calculating the elevation in boiling point (ΔT_b), as per the CBSE Class 12 syllabus for the 2025-26 session, is:
ΔT_b = i × K_b × m
- ΔT_b represents the elevation in boiling point.
- i is the van't Hoff factor, which indicates the number of particles the solute forms in the solution.
- K_b is the molal boiling point elevation constant, also known as the ebullioscopic constant, which is specific to the solvent.
- m is the molality of the solution, measured in moles of solute per kilogram of solvent.
3. How is the elevation in boiling point related to the lowering of vapour pressure?
The relationship is directly causal. A liquid boils when its vapour pressure equals the surrounding atmospheric pressure. When a non-volatile solute is introduced, its particles occupy surface space, hindering the escape of solvent molecules into the vapour phase. This causes a lowering of the vapour pressure of the solution. Consequently, the solution must be heated to a higher temperature to raise its vapour pressure to match the atmospheric pressure, resulting in an elevation of the boiling point.
4. Why is elevation in boiling point considered a colligative property?
Elevation in boiling point is classified as a colligative property because its magnitude depends on the concentration (number) of solute particles in the solution, rather than on their chemical identity or size. Whether the solute is sugar or salt, the elevation is proportional to the number of dissolved particles (ions or molecules). This dependency on the quantity, not the quality, of the solute is the defining characteristic of all colligative properties.
5. What is the ebullioscopic constant (K_b), and on what factors does it depend?
The ebullioscopic constant (K_b) is a proportionality constant that quantitatively connects the molality of a solution to the elevation in its boiling point. It is an intrinsic property that depends solely on the thermodynamic properties of the solvent, such as its molar mass, boiling point, and latent heat of vaporization. It does not depend on the nature of the solute added. Every solvent, like water, benzene, or ethanol, has its own unique K_b value.
6. Why is molality (m) used in the boiling point elevation formula instead of molarity (M)?
Molality is used because it is a temperature-independent measure of concentration. Boiling point calculations inherently involve changes in temperature. Molarity, defined as moles per litre of solution, is temperature-dependent because the volume of a solution can expand or contract with heat. In contrast, molality is defined as moles of solute per kilogram of solvent. Since mass is unaffected by temperature changes, molality provides a more stable and accurate value for calculations involving colligative properties.
7. What is the difference between elevation in boiling point and depression in freezing point?
Both are colligative properties that result from adding a non-volatile solute, but they impact phase transitions in opposite ways:
- Elevation in Boiling Point (ΔT_b): The boiling temperature of the solution becomes higher than that of the pure solvent. The solute lowers the vapour pressure, requiring more energy to boil.
- Depression in Freezing Point (ΔT_f): The freezing temperature of the solution becomes lower than that of the pure solvent. The solute particles interfere with the formation of the solvent's crystal lattice, requiring a lower temperature for it to solidify.
Essentially, adding a solute expands the temperature range over which the substance remains a liquid.
8. What are some real-world applications of boiling point elevation?
Boiling point elevation has several practical applications in daily life and industry:
- Automotive Antifreeze: The ethylene glycol in car coolants not only prevents freezing but also raises the boiling point of the water in the radiator, protecting the engine from overheating in hot conditions.
- Cooking: Adding salt or sugar to water increases its boiling point. This allows food to be cooked at a higher temperature, which can sometimes speed up the cooking process.
- Sugar Industry: In refining sugar, precise control of the boiling point of sucrose solutions is used to manage syrup concentration and promote the crystallisation of sugar.
